Portable isocapnia circuit and isocapnia method

ABSTRACT

A portable isocapnia circuit and an isocapnia method. A breathing port of the circuit allows a subject to inhale and exhale. Connected to the breathing port, a bifurcate conduit has a first conduit branch and a second conduit branch. The first conduit branch has an atmospheric air inlet, from which the atmospheric air is provided for inhalation, an inspiratory check valve to allow a one-way flow of the atmospheric air, and an atmospheric air aspirator. The second conduit branch has a one-way expiratory check valve and a expiratory gas reservoir. A one-way check valve is used to interconnect the first and second conduit branch, from which the expiratory gas stored in the expiratory gas reservoir flows to the first conduit branch to mix with the atmospheric air when the minute ventilation exceeds the atmospheric air.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims priority to Canadian Application SerialNo. 2,340,511 filed Mar. 12, 2001.

STATEMENT RE FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

[0002] (Not Applicable)

BACKGROUND OF THE INVENTION

[0003] The present invention relates to a portable isocapnia circuit anda method to set and stablize end tidal and arterial PCO₂ despite varyinglevels of minute ventilation.

[0004] Venous blood returns to the heart from the muscles and organspartially depleted of oxygen (O₂) and a full complement of carbondioxide (CO₂). Blood from various parts of the body is mixed in theright side of the heart (resulting in the formation of mixed venousblood) and pumped into the lungs. In the lungs the blood vessels breakup into a net of small vessels surrounding tiny lung sacs (alveoli). Thevessels surrounding the alveoli provide a large surface area for theexchange of gases by diffusion along their concentration gradients. Aconcentration gradient exists between the partial pressure of CO₂ (PCO₂)in the mixed venous blood (PvCO₂) and that in the alveolar PCO₂. The CO₂diffuses into the alveoli from the mixed venous blood from the beginningof inspiration until an equilibrium is reached between the PvCO₂ and thealveolar PCO₂ at some time during breath. When the subject exhales, thefirst gas that is exhaled comes from the trachea and major bronchi whichdo not allow gas exchange and therefore will have a gas compositionsimilar to that of inhaled gas. The gas at the end of exhalation isconsidered to have come from the alveoli and reflects the equilibriumCO₂ concentration between the capillaries and the alveoli. The PCO₂ inthis gas is the end-tidal PCO₂ (PETCO₂).

[0005] When blood passes the alveoli and is pumped to the left side ofthe heart to the arteries in the rest of the body, it is known as thearterial PCO₂ (PaCO₂). The arterial blood has a PCO₂ equal to the PCO₂at equilibrium between the capillaries and alveoli. With each breathsome CO₂ is eliminated from the lung and fresh air containing little orno CO₂ (CO₂ concentration is assumed to be 0%) is inhaled and dilutesthe residual alveolar PCO₂, establishing a new gradient for CO₂ todiffuse out of the mixed venous blood into the alveoli. The flow offresh gas in and out of the lungs each minute, or minute ventilation(V), expressed in L/min, is that required to eliminate the CO₂ broughtto the lungs and maintain an equilibrium PCO₂ and (PaCO₂) ofapproximately 40 mmHg (in normal humans). When one produces more CO₂(for example, as a result of fever or exercise), more CO₂ is producedand carried to the lungs. When CO₂ production is normal, the PaCO₂ fallsif one increases the ventilation (hyperventilation). On the contrary,when CO₂ production remains normal, the PaCO₂ rises if one increases theventilation (hypoventilation).

[0006] It is important to note that not all V contributes to eliminationof CO₂. Some V goes air passage (trachea and major bronchi) and alveoliwith little blood perfusing them, and thus contributes minimally toeliminating CO₂. This V is termed “dead space” ventilation and gas inthe lung that has not participated in gas exchange with the blood iscalled “dead space” gas. That portion of V that goes to well-perfusedalveoli and participates in gas exchange is called the alveolarventilation (VA) and exhaled gas that has participated in gas exchangein the alveoli is termed “alveolar gas”.

[0007] A method of accelerating the resuscitation of a patient has beendisclosed in PCT application No. WO98/41266 filed by Joe Fisher. Whenthe patient breathes at a rate such that his ventilation is less than orequal to the fresh gas flowing into the fresh glowing into the circuit,all of the inhaled gas is made up of fresh gas. When the patient'sminute ventilation exceeds the fresh gas flow, the inhaled gas is madeup of all of the fresh gas and the additional gas is provided by“reserve gas” consisting of fresh gas plus CO₂ such that theconcentration of CO₂ in the reserve gas of about 6% has a PCO₂ equal tothe PCO₂ in the mixed venous blood.

[0008] The limitation of the above method is that a source of reservegas and its delivery apparatus must be supplied to pursue the method.The reserve gas must be at about 6% of CO₂ concentration substantiallyhaving a PCO₂ equal to that of mixed venous blood or about 46 mmHg. Theportability is thus limited since a sufficiently long tubular structure,typically about 3 m, is required to prevent the atmospheric air fromdiffusing in and diluting the expired CO₂ concentrations. While climbingat high altitude, it would be very difficult to carry oxygen tanks and along tubular expired gas reservoir. Another example of such a difficultywould be when preventing hyperventilation while ventilating with air inthe course of resuscitating newborns and adults in out-of-hospitalsetting.

SUMMARY OF THE INVENTION

[0009] The invention provides a isocapnia circuit that maintains aconstant PCO₂. A flexible container is used to replace the fresh gasreservoir bag used in the prior art. The flexible container is activelycollapsed by the inspiratory effort of the patient during inspirationand passively expands during expiration, the atmospheric air is thusdrawn into the flexible container and the circuit through a port. Theexpiratory reservoir is provided with a flexible bag so that the volumeof expired gas rebreathed is displaced by collapse of the bag ratherthan entrainment of atmospheric air, thus preventing the dilution of CO₂in the expired gas reservoir.

[0010] Therefore, the benefits of controlling the PCO₂ at a constantlevel are reaped and the expense and inconvenience of supplying freshgas are not incurred. The compact nature of the isocapnia circuit makesits use practical outdoors, during physical activity and in remoteenvironments. It has been determined that people living at high altitudesuch as mountaineers, miners, astronomical observatory personnel wouldbenefit from preventing the PCO₂ level falling excessively as a resultof the involuntary tendency to hyperventilate while they are at highaltitude. It would also been determined that resuscitation of newbornswith air has demonstrable advantages over resuscitation with oxygen ifexcessive decrease in PCO₂ can be prevented. This therefore was notcontemplated in the prior art.

[0011] In the invention, the PCO₂ is controlled at a predetermineddesired level without the need of gas from another source flowing intothe circuit under pressure. The expired gas is stored to preventdilution with atmospheric air such that alveolar portion of the expiredgas is rebreathed in preference to dead space gas. The improvedbreathing circuit can be used to assist patients who are or run the riskof suffering the effects of high altitudes sickness, or who havesuffered a cardiac arrest, or who have suffered from an interruption ofblood flow to an organ or region of an organ and are at risk ofsuffering oxidative injury on restoration of blood perfusion as wouldoccur with a stroke or heart attack or resuscitation of the newborn.

[0012] In the method of providing a constant PCO₂. atmospheric air isaspirated from an inspiratory side to a patient when the patient inhalesthrough the inspiratory side. A gas exhaled by the patient isaccumulated in an expiratory gas reservoir connected to an expiratoryside, through which the patient exhale. The gas exhaled by the patientand stored in the expiratory gas reservoir is allowed flowing into theinspiratory side to mix with the aspirated atmospheric air when a minuteventilation of the patient exceeds the atmospheric air aspirated to theinspiratory side.

[0013] The above isocapnia circuit comprises a breathing port, throughwhich a subject inhales and exhales; an inspiratory port, communicatingto the breathing port with an inspiratory valve that allows air flowingto the breathing port and prevents air flowing from the breathing portto the inspiratory port, the inspiratory port having an atmospheric airaspirator to aspirate the atmospheric air therein; an expiratory port,communicating to the breathing port with an expiratory valve that allowsair flowing from the breathing port to the expiratory port and preventsair flowing to the breathing port, the expiratory port having anexpiratory gas reservoir to store a gas exhaled by the subject flowingacross the expiratory valve; and a bypass conduit, communicating theinspiratory and expiratory ports with a bypass valve, the bypass valveallows a one-way flow of air from the expiratory port to the inspiratoryport with a pressure differential applied thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 schematically shows an isocapnia circuit in one embodimentof the invention; and

[0015]FIG. 2 and FIG. 3 show data resulting from the method and thecircuit applied in the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0016]FIG. 1 shows an air isocapnia circuit. The isocapnia circuit has aY-piece 11, through one breathing port of which, the patient or subjectbreathes (inhales and exhales). In addition to the breathing port, theisocapnia circuit has another two ports in a form of two limbs of theY-piece 11, and each of them comprises a one-way valve. One of the limbswith an inspiratory valve 12 functions as an inspiration port, while theother limb with an expiratory valve 13 functions as an expiration port.The inspiratory valve 12 directs gas to flow towards the patient whenthe patient makes an inspiratory effort, and acts as a check valvepreventing flow in the opposite direction during exhalation. Theexpiratory valve 13 allows gas to exit the Y-piece 11 when the patientexhales, and also acts as a check valve to prevent flow towards thepatient when the patient inhales.

[0017] Distal to the expiratory limb of the Y-piece 11, a large boretubing termed “alveolar gas reservoir” 14 is attached. The alveolar gasreservoir 14 is contained in a pliable bag of about 3 L in volume. Thepliable bag, named “expiratory gas reservoir bag” 15, has a proximal endsealed around a proximal end of the alveolar gas reservoir 14. Theexpiratory gas reservoir bag 15 further has another tubing called the“exhaust tubing” 16 situated at a distal end where the expired gas exitsto atmosphere 17. Thus arranged, most of the exhaust tubing 16 iscontained in the expiratory gas reservoir bag 15, and which is sealed tothe circumference of the exhaust tubing 16 at its distal end.Preferably, the exhaust tubing 16 has a diameter smaller than that ofthe alveolar gas reservoir 14. In one embodiment, the alveolar gasreservoir 14 is about 35 mm in diameter, and has a length to provide atotal volume of about or greater than 0.3 while being applied to anaverage (70 kg) adult. The inspiratory limb opens into a cylindricalcontainer comprising a rigid proximal end plate 18, a collapsibleplicate tube 19 extending distally from the circumference of theproximate end plate 18, and a distal end rigid plate 20 sealing thedistal end of the collapsible plicate tube 19. When not in use, thecollapsible plicate tube 19 is kept open by the gravitation of thedistal end rigid plate 20, and/or by the force of a spring 21 attachedon the collapsible plicate tube 19, and/or by intrinsic recoil of theplicate tubing 19. The inspiratory limb is also open to the atmosphereby means of a nozzle 22, to which a tube 23 is attached. The rigid plate20 is open to a nozzle 24, to which another tube 25 is attached. Theproximal end plate 18 has a protuberance 26 pointing at the tube 25 thatis aligned with the internal opening of the distal end plate nozzle 24.The combination of the proximal end plate 18, the collapsible plicatetube 19, the distal end rigid plate 20, the spring 21, the inspiratorylimb nozzle 22, the tube 23 attached to the nozzle 22, the distal endplate nozzle 24, the tube 25 attached to the distal end plate nozzle 24and the protuberance 26 are in aggregate as am “atmosphere air aspirator(AAA)”. A bypass conduit 27 is further included in the Y-piece 11 toconnect the expiratory limb and the inspiratory limb. The opening of thebypass conduit 27 is preferably as close as possible to the expiratoryvalve 13. The bypass conduit 27 has a one-way valve 28 allowing flowfrom the expiratory limb to the inspiratory limb only. The one-way valve28 of the bypass conduit 27 requires an opening pressure differentialslightly greater than the pressure difference between the inspiratorylimb pressure and atmospheric pressure that is sufficient to collapsethe plicate tube 19. In this way, during inspiration, atmosphere aircontained in the atmospheric air aspirator and the air beingcontinuously aspirated into the inspiratory limb is preferentially drawnfrom the inspiratory manifold.

[0018] Considering the above isocapnia circuit without the spring 21,the nozzle 24 on the distal end plate 24, or the internally directedprotuberance 26, each inspiration drawn initially from the atmosphericair aspirator collapses the plicate tube 19 and approximates the distalend plate 20 to the proximal end plate 18 when the patient begins tobreathe. As long as the plicate tube 19 is partially collapsed, there isa constant sub-atmospheric pressure in the inspiratory limb of theisocapnia circuit. The sub-atmospheric pressure creates a pressuregradient that draws the atmospheric air into the inspiratory limb of theisocapnia circuit through the nozzle 22 and the tube 23. When the minuteventilation of the subject is equal to or less than the intended flow ofatmospheric air into the aspirator, only atmospheric air is breathed.During exhalation, atmospheric air accumulates in the aspirator. Duringinhalation, inspired gas consist of the contents of the atmospheric airaspirator and the atmospheric air flowing into the inspiratory limbthrough the nozzle 23. When the minute ventilation of the subjectexceeds the net flow of the atmospheric air into the isocapnia circuit,air is breathed for each breath until the atmospheric air aspirator iscollapsed. Additional inspiratory efforts result in an additionaldecrease in gas pressure on the inspiratory side of the isocapniacircuit.

[0019] When the pressure differential across the valve 28 of the bypassconduit 27 exceeds its opening pressure, the one-way valve 28 opens andthe exhaled gas is drawn back from the expiratory reservoir bag 15 intothe inspiratory limb of the Y-piece 11 and hence to the patient. To theextent that the opening pressure of the valve 28 is close to thepressure generated by the recoil of the atmospheric air aspirator, therewill be little change in the flow of atmospheric air into the isocapniacircuit during inspiration after the atmospheric air aspirator hascollapsed. The last gas to be exhaled during the previous breath, termed“alveolar gas”, is retained in the alveolar gas reservoir 14 and is thefirst gas to be drawn back into the inspiratory limb of the isocapniacircuit and inhaled (rebreathed) by the patient. After several breaths,the rest of the expired gas from the expiratory gas reservoir bag 15contains mixed expired gas. The mixed expired gas from the expiratorygas reservoir bag 15 replace the gas drawn from the alveolar gasreservoir 14 and provides the balance of the inspired volume required tomeet the inspiratory effort of the patient. The greater restriction inthe diameter of the second tube, that is, the exhaust tubing 16, than inthe alveolar gas reservoir 14 results in the gas being drawn into thealveolar gas reservoir 14 being displaced by the collapse of theexpiratory gas reservoir bag 15 in preference to drawing air from theambient atmosphere. The exhaust tubing in the expiratory reservoir bag16 provides a route for exhaust of expired gas and acts as a reservoirfor that volume of atmospheric air diffusing into the expiratoryreservoir bag through the distal opening, tending to keep suchatmospheric air separate from the mixed expired gas contained in theexpiratory gas reservoir bag 15.

[0020] During exhalation and all of inhalation until the collapse of theatmospheric gas aspirator, the flow of atmospheric air into the circuitwill remain constant. However, after the atmospheric air aspiratorcollapses, the pressure gradient will increase. The effect of theincrease in total flow will depend on the difference between the openingpressure of the bypass valve 28 and the recoil pressure of theatmospheric air aspirator times the fraction of the respiratory cyclewhen the atmospheric air aspirator is collapsed. If the fraction of therespiratory cycle when the atmospheric air aspirator is collapsed isgreat, as when there is a very great excess minute ventilation above therate of atmospheric air aspiration, the atmospheric air aspirator can bemodified adding a second port for air entry at, for example, the distalend plate nozzle 24. As a result, the total flow from the two portsprovides the desired total flow of air into the circuit under the recoilpressure of the atmospheric air aspirator. When the atmospheric airaspirator collapses on inspiration, the second port 24 is occluded bythe protuberance 26. The remaining port, that is, the nozzle 22,provides a greater resistance to air flow to offset the greater pressuregradient being that gradient required to open the bypass valve 28.

[0021] In the above embodiment, it is assumed that the gravitationacting on the distal plate 20 provides the recoil pressure to open theatmospheric air aspirator. The disadvantage to this configuration isthat the distal end plate 20 must be heavy enough to generate thesub-atmospheric pressure. This may be too heavy to be supported byattachment to a face mask strapped to the face. Furthermore, movementsuch as walking or running or spasmodic inhalation will cause variationsin the pressure inside the atmospheric air aspirator and hence variationin flow of air into the atmospheric air aspirator. In such cases, it isbetter to minimize the mass of the distal end plate 20 and use adifferent type of motive force to provide recoil symbolized by thespring 21.

[0022] The advantages of the isocapnia circuit and method provided bythe invention in terms of the operation and portability are clearlyevident from the data charted in FIGS. 2 and 3, in which levels ofminute ventilation, expired gas flow and airway PCO₂ are compared.

[0023] Preferably, the above isocapnia circuit is installed in a case torender it fully portable. Ths case may include the appropriate number ofcapped ports to allow proper set up and use of the isocapnia circuit.

[0024] The operation of the above isocapnia circuit is described asfollows. When the minute ventilation for the patient breathing throughthe breathing port is equal to the rate of atmospheric air aspiratedinto the isocapnia circuit, for example, 5 L per minute, the atmosphericair enters the breathing port from the first conduit branch, that is,the inspiratory limb, at a predetermined rate, preferably, 5 L perminute and is exhaled through the second conduit branch, that is, theexpiratory limb, at a rate of preferably 5 L per minute. The exhaled gastravels down to the expiratory gas reservoir bag 25. When the minuteventilation exceeds the aspirated atmospheric air, the patient inhalesthe expiratory gas retained in the expiratory gas reservoir bag 25. Theexpiratory gas passes through the bypass valve 28 to the inspiratorylimb to make up the shortfall of the atmospheric air. In this way, thePCO₂ is maintained with a constant value even when the minuteventilation increases.

[0025] To maintain the constant value of PCO₂, it is important to haveatmospheric air aspirator depleted of gas first until it is just emptyat the end of an inhalation cycle. Once the minute ventilation isincreased, the increased breathing effort will further decrease thesub-atmospheric pressure in the inspiratory limb and open the bypassvalve 28 to allow the expiratory gas supplied as a part of the breathinggas during the entire breathing cycle.

[0026] During resuscitation of an asphyxiated newborn or an adultsuffering a cardiac arrest, the blood flow through the lungs isremarkably slow during resuscitation attempts. Even normal rates ofventilation may result an excessive elimination of CO₂ from the blood.As the blood reaches brain, the low PCO₂ may constrict the blood vesselsand limit the potential blood flow to the ischemic brain. By attachingthe isocapnia circuit provided by the invention to the gas inlet port ofa resuscitation bag and diverting all expiratory gas to the expiratorygas reservoir bag, the decrease of PCO₂ would be limited.

[0027] The above isocapnia circuit can be applied to enhance the resultsof a diagnostic procedure or a medical treatment including the followingsteps. The circuit without a source of forced gas glow and capable oforganizing exhaled gas is used. With the circuit, preferentialrebreathing of alveolar gas in preference to dead space gas is providedwhen the patient is ventilating at a rate greater than the rate ofatmospheric air aspirated, and when inducing hypercapnia is desired. Bydecreasing the rate of aspirated atmospheric air, a correspondingincrease in rebreathed gas is passively provided to prevent the PCO₂level of arterial blood from dropping despite increase in minuteventilation. The step of inducing hypercapnia is continued until thediagnostic or medical therapeutic procedure is complete. The results ofthe diagnostic or medical procedure are thus enhanced by carrying outthe method in relation to the results of the procedure had the methodnot been carried out. Examples of such procedures include MRI orpreventing spasm of brain vessels after brain hemorrhage, radiationtreatments or the like.

[0028] The invention can also be applied to treat or assist a patient,preferably human, during a traumatic event characterized byhyperventilation. A circuit that does not required a source of forcedgas flow, in which alveolar ventilation is equal to the rate ofatmospheric air aspirated and increases in alveolar ventilation withincreases in minute ventilation is prevented, is provided. The circuit,for example, the isocapnia circuit as described above, is capable oforganizing exhaled gas provided to the patient preferential rebreathingalveolar gas in preference to dead space gas following ventilating thepatient at a rate of normal minute ventilation, preferably approximately5L per minute. When desired, hypercapnia is induced to increase arterialPCO₂ and prevent the PCO₂ level of arterial blood from dropping. Thenormocapnia is maintained despite the ventilation is increased until thetraumatic hyperventilation is complete. As a result, the effects ofhyperventilation experienced during the traumatic event are minimized.This can be applied when the mother is in labor and becomes light headedor the baby during the delivery is effected with the oxygen delivery toits brain being decreased as a result of contraction of the bloodvessels in the placenta and fetal brain. A list of circumstances inwhich the method enhancing the diagnostic procedure results or theexperience of the traumatic even are listed below.

[0029] Applications of the method and circuit includes:

[0030] 1) Maintenance of constant PCO₂ and inducing changes in PCO₂during MRI.

[0031] 2) Inducing and/or marinating increased PCO₂:

[0032] a) to prevent or treat shivering and tremors during labor,post-anesthesia, hypothermia, and certain other pathological states;

[0033] b) to treat fetal distress due to asphyxia;

[0034] c) to induce cerebral vasodilatation, prevent cerebral vasospasm,and provide cerebral protection following subarachnoid hemorrhagecerebral trauma and other pathological states;

[0035] d) to increase tissue perfusion in tissues containing cancerouscells to increase their sensitivity to ionizing radiation and deliveryof chemotherapeutic agents;

[0036] e) to aid in radiodiagnostic procedures by providing contrastbetween tissues with normal and abnormal vascular response; and

[0037] f) protection of various organs such as the lung, kidney andbrain during states of multi-organ failure.

[0038] 3) Prevention of hypocapnia with O₂ therapy, especially inpregnant patients.

[0039] 4) Other applications where O₂ therapy is desired and it isimportant to prevent the accompanying drop in PCO₂.

[0040] When minute ventilation is greater than or equal to the rate ofatmospheric air aspirated, the above-mentioned preferred circuit ensuresthat the patient receives all the atmospheric air aspirated into thecircuit independent of the pattern of breathing since atmospheric airalone enters the fresh gas reservoir, and exhaled gas enters its ownseparate reservoir and all the aspirated air is delivered to the patientduring inhalation before rebreathed exhaled gas. The atmospheric airaspirator is large enough not to fill to capacity during a prolongedexhalation when the total minute ventilation exceeds the rate ofatmospheric air aspiration ensuring that under these circumstancesatmospheric air continues to enter the circuit uninterrupted duringexhalation. The preferred circuit prevents rebreathing at a minuteventilation equal to the rate of air being aspirated into theatmospheric air aspirator because the check valve in the interconnectingconduit does not open to allow rebreathing of previously exhaled gasunless a sub-atmospheric pressure less than that generated by the recoilof the aspirator exists on the inspiratory side of the conduit of thecircuit. The circuit provides that after the check valve opens, alveolargas is rebreathed in preference to dead space gas because theinterconnecting conduit is located such that exhaled alveolar gascontained in the tube conducting the expired gas into the expiratoryreservoir bag will be closest to it and dead space gas will be mixedwith other exhaled gases in the reservoir bag. The exhaled gas reservoiris preferably sized at about 3 L which is well excess of the volume ofan individual's breath. When the patient inhales gas from the reservoirbag, the reservoir bag collapses to displace the volume of gas extractedfrom the bag, minimizing the volume of atmospheric air entering the bag.

[0041] The basic approach of preventing a decrease in PCO₂ with increaseventilation is briefly described as follows. Only breathing the freshgas contributes to alveolar ventilation (VA) which establishes thegradient for CO₂ elimination. All gas breathed in excess of the freshgas entering the circuit, or the fresh gas flow, is rebreathed gas. Thecloser the partial pressure of CO₂ in the inhaled gas to that of mixedvenous blood (PvCO₂), the less the effect on CO₂ elimination. Therelationship between the alveolar ventilation, minute ventilation (V)and PCO₂ of rebreathed gas is VA=FGF+(V−FGF)(PvCO₂−PCO₂ of exhaledgas)/PvCO₂ where FGF stands for the fresh gas flow. With respect to thiscircuit, the fresh gas flow is equivalent to the rate of atmospheric airaspirated into the atmospheric air aspirator.

[0042] It is clear from this equation that as the PCO₂ of the exhaledgas approaches that of the mixed venous blood, the alveolar ventilationis determined only by the fresh gas flow and not the minute ventilation.

[0043] As one exhales, the first gas to exit the mouth comes from thetracheas where no gas exchange has occurred. The PCO₂ of this gas isidentical to that of the inhaled gas and is termed as “dead space gas”.The last gas to exit the mouth originates from the alveoli and has hadthe most time to equilibrate with mixed venous blood, has a PCO₂ closestto that of mixed venous blood and is termed as “alveolar gas”. Gasexhaled between these two periods has a PCO₂ intermediate between thetwo concentrations. The equation cited above explains why rebreathingalveolar gas would be the most effective in maintaining the PCO₂ at aconstant level when minute ventilation increases.

[0044] Accordingly, in the above isocapnia circuit:

[0045] 1. All of the fresh gas, in the form of atmospheric air, isinhaled by the subject and contributes to alveolar ventilation whenminute ventilation is equal to or exceeds the rate of atmospheric airaspirated into the AAA.

[0046] 2. The alveolar gas is preferentially rebreathed when minuteventilation exceeds the fresh gas flow.

[0047] Other embodiments of the invention will appear to those skilledin the art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples to be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

What is claimed is:
 1. A method of providing constant PCO₂ to a patient wearing a breathing port having an inspiratory side and an expiratory side, the method comprising: aspirating atmospheric air from the inspiratory side to a patient when the patient inhales through the inspiratory side; accumulating the gas exhaled by the patient in an expiratory gas reservoir connected to the expiratory side, through which the patient exhales; and allowing the gas exhaled by the patient and stored in the expiratory gas reservoir to flow into the inspiratory side to mix with the aspirated atmospheric air when a minute ventilation of the patient exceeds the atmospheric air aspirated to the inspiratory side.
 2. An isocapnia circuit, comprising: a breathing port, through which a subject inhales and exhales; an inspiratory port, communicating to the breathing port with an inspiratory valve that allows air flowing to the breathing port and prevents air flowing from the breathing port to the inspiratory port, the inspiratory port having an atmospheric air aspirator to aspirate the atmospheric air therein; an expiratory port, communicating to the breathing port with an expiratory valve that allows air flowing from the breathing port to the expiratory port and prevents air flowing to the breathing port, the expiratory port having an expiratory gas reservoir to store gas exhaled by the subject flowing across the expiratory valve; and a bypass conduit, communicating the inspiratory and expiratory ports with a bypass valve, the bypass valve allows a one-way flow of air from the expiratory port to the inspiratory port with a pressure differential applied thereto.
 3. The isocapnia circuit as claimed in claim 2, wherein the atmospheric air aspirator further comprises: a first end plate, where the inspiratory port opens to; a collapsible plicate tube, a second end rigid plate, with the collapsible tube accommodated between the first end plate and the second end rigid plate; an inspiratory port nozzle located between the inspiratory valve and the first end plate, where the inspiratory port opens to the atmospheric air; a first tube, attached to the inspiratory port nozzle; an end plate nozzle, from which the collapsible tube opens to the atmospheric air; a second tube, attached to the end plate nozzle; and a protuberance, attached on the first end plate and pointing at the end plate nozzle to close the opening of the collapsible plicate tube while collapsed.
 4. The isocapnia circuit as claimed in claim 3, wherein the atmospheric air aspirator further comprises a spring to recoil the collapsible plicate tube.
 5. The isocapnia circuit as claimed in claim 2, wherein the expiratory gas reservoir further comprises: a alveolar gas reservoir, connecting the expiratory port and the expirator gas reservoir, around one end of which the expiratory gas reservoir being sealed, the alveolar gas reservoir has the other end extending into the expiratory the gas reservoir; and an exhaust tubing, from which the gas within the expiratory gas reservoir exhausts.
 6. The isocapnia circuit as claimed in claim 5, wherein the alveolar gas reservoir has a diameter larger than that of the exhaust tubing.
 7. The isocapnia circuit as claimed in claim 2, wherein the expiratory gas reservoir has a capacity in excess of a volume of a user's breath.
 8. An isocapnia circuit, comprising: a breathing port, through which a subject exhales and inhales; a bifurcated conduit adjacent and connected to the breathing port, including a first conduit branch and a second conduit branch, the first conduit branch further including: an atmospheric air inlet; and an inspiratory check valve, located between the breathing port and the atmospheric air inlet, wherein the inspiratory and expiratory check valves are both one-way passage vales, and the second conduit branch further including an expiratory check valve; an atmospheric air aspirator connected to the first conduit branch, having a collapsible container formed to recoil to an open position; a flexible expiratory gas reservoir, having an entrance tubing through which the flexible expiratory gas reservoir is connected to the second conduit, and an exit tubing open to the atmospheric air; and a bypass conduit communicating between the first and the second conduit branches, having a one-way check valve therein. 